This invention relates to methods and apparatuses for fractionating or separating gases, more particularly to such methods and apparatuses which operate cyclically using adsorbant material.
Heat-reactivated desiccant dehydrators (temperature-swing desiccant dehydrators) generally operate on a four-hour cycle in which two towers are alternately on-line and reactivated. They are reactivated by heating, alternately, the entire bed of each tower to 400.degree. F. to 800.degree. F. and then cooling them back to near the inlet air temperature. The long cycles depend on an efficient, high capacity desiccant and a long, hot purge cycle to desorb the bed. In order to provide this heat, large internal and/or external heaters are required. These high temperature heaters cause many reliability problems and reduce desiccant life, among other disadvantages. Also, because the desiccant is a good insulation material, the heat is not distributed well and the entire towers may not be fully reactivated. Where short operating periods (on the order of less than one hour) are required, complete reactivation of a tower is prevented from occurring.
Pressure-swing desiccant dehydrators have been considered and attempted but have not been successfully demonstrated for high pressure, short-cycle operations. Pure pressure-swing dehydrators operate without the external addition or removal of heat and depend on a short cycle, on the order of minutes or seconds. In such systems, only the outer layer of the desiccant particle material effectively adsorbs water vapor, desorption being accomplished by passing dried low-pressure air through the bed. This desiccant regeneration method does not desorb adequately enough water vapor to meet stringent dehydrator performance requirements. In drying air, desorption is further hampered by the Joule-Thomson cooling effect which occurs when high-pressure air is depressurized and by the endothermic cooling effect which occurs with water vapor desorption.
Skarstrom U.S. Pat. No. 2,944,627 discloses the fractionating of a gaseous mixture by adsorption; in specific application, Skarstrom discloses the drying of a gaseous mixture such as air by removal of water vapor.
Cassano U.S. Pat. No. 4,565,685 discloses the use of a combination of temperature swing and pressure swing absorption-desorption sequence procedures to cyclically provide a continuous separation of the components of air, and specifically oxygen, from air.
Seibert et al. U.S. Pat. No. 3,448,561 and McKey et al. U.S. Pat. No. 4,127,395 disclose method and apparatus for adsorbing water vapor from a mixture thereof with a second gas to reduce the water vapor concentration in the mixture to below a permissible maximum concentration. The Seibert et al. reference and the McKey et al. reference each disclose a process which includes removing sorbed water vapor from a bed of a sorbent at a pressure below the pressure at which adsorption is effected or at an elevated temperature sufficient to desorb the water vapor.
White, Jr. et al. U.S. Pat. No. 4,197,095 disclose method and apparatus for adsorbing one or more first gases from a mixture thereof with a second gas to reduce the concentration of the first gas(es) in the mixture to below a permissible maximum concentration.
Seibert et al., McKey et al. and White, Jr. et al. disclose applicability of their respective inventions to systems wherein no heat is applied to the desiccant bed to effect regeneration, to systems wherein part or all of the desiccant bed is heated to effect regeneration, to systems wherein regeneration is effected at reduced pressure, to systems utilizing a purge-gas flow, and to systems combining one or more of these features.